Influence of oxygen flux on properties of nickel oxide films prepared by magnetron sputtering for alternative hole-transporting layer in inverted perovskite solar cells

Sputtered nickel oxide (NiO x ) films applied as hole-transporting layer in inverted perovskite solar cells have attracted tremendous attentions recently due to its wide band gap, good process compatibility with large-scale fabrication, and good chemical stability. However, the conductivity of sputtered NiO x films is relatively low. In this study, a trace amount of oxygen was introduced to improve the conductivity of NiO x films, and the oxygen flux will affect the transparency and conductivity of NiO x films during sputtering process, which is vital to its application in inverted perovskite solar cells. Therefore, the effects of different oxygen fluxes on the optical and electrical properties, microstructure, and surface morphology of NiO x films prepared by radio-frequency (RF) magnetron sputtering at room temperature were investigated. The NiO x films deposited under different oxygen fluxes showed polycrystalline structures, and the introduction of oxygen gas led to a (111) preferential orientation and reduced grain size. With increased oxygen flux, we obtained NiO x films with smaller grain size and lower roughness, besides, the transmittance decreased and a slight red shift of the band gap was observed. Hall measurements showed the sputtered NiO x films have p-type semiconductor properties, the resistivity decreased from 2.16 × 10 4 to 5.31 × 10 2 Ω cm and the hole concentration increases from 4.25 × 10 11 cm −3 to 5.35 × 10 14 cm −3 with increasing oxygen flux from 0.05 to 0.8%. Sputtered NiO x films with improved conductivity were successfully deposited, and the results showed the effect of oxygen flux on the properties of sputtered NiO x films, paving the way for research on high efficiency NiO-based IPSCs.